The present invention relates to a valve positioner for controlling a valve which affects a process variable. More specifically, the present invention relates to a valve positioner having a microprocessor.
Various types of positioners are used in the process control industry. Some positioners are mechanically coupled to an actuator while some incorporate the positioner within the actuator. The actuator provides means for physically positioning the valve and may be electric, hydraulic or pneumatic. Electric actuators have a current signal which drives a motor which positions the valve. Hydraulic actuators have oil-filled means for positioning the valve. By far the most common in the process control industry, a pneumatic actuator has a piston or a combination of a spring and diaphragm which position the valve. Depending on the application and the level of control integration, positioners receive several types of input from a controller which are representative of the desired valve position. One type is a current input having a 4-20 mA or a 10-50 mA magnitude, a second is a digital signal superimposed on the current signal and a third is a fully digital input such as Fieldbus or Hodbus.RTM.. Alternatively, the positioner may receive a 3-15 or 6-30 pound per square inch (PSI) pneumatic input representative of the desired valve position. Depending on the level of integration and the application as well, positioners have different types of outputs. Some positioners provide an output current to an electric actuator motor, while still others have a fast responding hydraulic output. The most common type of positioner output is a 0-200 PSI pneumatic output. Positioner, as the word is used in this application, includes all these field mounted instruments, including the various inputs and outputs, and their respective means for positioning valves, if applicable. U.S. Pat. Nos. 4,665,938 and 5,197,328 are examples of valve positioners.
In the most common case of a spring and diaphragm actuator, the diaphragm deflects with the pressure delivered by the positioner, thereby exerting a force or torque on a control valve stem or rotary member, respectively, so as to change the position of the valve. Many positioners have a mechanical or an electronic position sensor to provide a position signal which is fed back into a microprocessor-based control section of the positioner. No matter what the specific means are for delivering force to position a valve, positioners having microprocessors are known. Existing microprocessor-based positioners provide good loop dynamic response, but have a limited band-width so that their usage is limited to slow control loops such as one which controls level in a tank or temperature in a reactor.
With the introduction of microprocessors into valve positioners, the microprocessor is being called on to perform increasingly complex functions, resulting in higher overall power requirements for the positioner. In control systems using, for example, the 4-20 mA, 10-50 mA, Fieldbus and Modbus.RTM. protocols, the operating power of the positioner is limited, since the positioner operates exclusively on the power supplied by the control loop. This limitation has placed an upper limit on the amount of computation and the clock frequency (dynamic response) of the microprocessor in the valve positioner. As technology has advanced, it has been possible to obtain more computational power from a microprocessor without increasing the electrical power requirements. However, the desire to increase control and performance of valve positioners has outpaced the ability of the microprocessor to deliver the requirements under the aforementioned electrical power constraints. A low Dower positioner with wide bandwidths under computer control is needed.